JP4088558B2 - Overcurrent protection circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an overcurrent protection circuit used for an electronic device such as a wireless device, and in particular, to quickly stop an overcurrent when a load is short-circuited (short-circuited) to suppress the occurrence of a spark. It relates to the protection circuit.
[0002]
[Prior art]
Conventionally, as a battery used for an electronic device such as a wireless device, not only a primary battery such as an alkaline battery but also a secondary battery pack such as a rechargeable nickel-cadmium (Ni-Cd) battery pack has been used. Often.
When the load on the electronic device side is short-circuited on the electronic device side in which these battery packs are installed, there is a problem that an excessive current flows and sparks are generated or the battery pack is damaged. To do.
In particular, when flammable gas is present in the usage environment of the electronic device, there is a risk that the flammable gas may explode due to the generated spark. In such a use environment, it is desired to suppress the occurrence of sparks by limiting overcurrent.
For this reason, various protection devices and circuits for battery packs have been proposed in Japanese Patent Application Laid-Open No. 5-174874 (Patent Document 1).
[0003]
[Patent Document 1]
JP-A-5-174874 [0004]
The protection device described in Patent Document 1 limits voltage detection means that generates an overload signal when a load voltage applied to a load is a predetermined value or less, and limits a current flowing through the load when the overload signal is received. Specifically, as shown in FIG. 3, a protection device for a storage battery pack including a current control unit includes a transistor 141 as a current control unit, a transistor 142, and a resistor 143 that divides a load voltage. , 144 and the resistors 145 and 146 constitute a protective device 140.
In the protective device described in Patent Document 1, when the load 120 is normal, the load voltage at point A is within the rated voltage range. In this case, the voltage at the point B divided by the resistors 143 and 144 is sufficient to make the transistor 142 conductive. Accordingly, the potential at the point C becomes a low value, a base current flows through the emitter-base of the transistor 141, the resistor 145, the transistor 142, and the resistor 146, and the transistor 141 is kept conductive.
For example, when the load 120 is short-circuited and an excessive load current flows, the output voltage, that is, the voltage at the point A depends on the magnitude of the load current due to the characteristics of the battery pack 110 that is a secondary battery. Decline. When the voltage at the point A falls below a predetermined value, the voltage at the point B falls below the minimum voltage for conducting the transistor 142 due to the preset resistance ratio of the voltage dividing resistors 143 and 144.
Accordingly, the potential at the point C is increased, the transistor 141 is turned off, and the load current does not flow. That is, the potential at point C can be regarded as an overload signal indicating an overload state.
In this way, the short state of the load 120 is not detected by the temperature rise but is electrically detected, so that an excessive current can be dealt with instantaneously. Therefore, according to the protection device described in Patent Document 1, it is possible to prevent the occurrence of a spark due to a short circuit and to protect the battery pack without providing a switch inside the battery pack.
[0005]
[Problems to be solved by the invention]
However, in recent years, at the time of filing of Patent Document 1 and the like, a storage battery (secondary battery) having a large capacity and output that was not used in a wireless device or the like, for example, a lithium ion (Li-ion) battery has been developed. It has been put into practical use and has come to be used for wireless devices and the like.
For this reason, the protection device also operates at high speed and reliably, and a safer device has been required.
In particular, since a lithium ion battery has a large current capacity, a current increases and a large current flows in a very short time after the load is short-circuited, and sparks are likely to occur.
Therefore, it is necessary to quickly detect that the load is short-circuited, and to cut off the current flowing to the load in an extremely short time of about several hundred nanoseconds before the current increases and sparks are generated. There is.
However, since the protection device described in Patent Document 1 cannot sufficiently cope with such a high speed, a protection device capable of higher speed operation is required.
Therefore, the present invention has been made in view of the above circumstances, and by preventing an overcurrent when a load is short-circuited (short-circuit) in an extremely short time, it is possible to prevent occurrence of a spark and reliably It is an object of the present invention to provide an overcurrent protection circuit that can protect a battery pack.
[0006]
[Means for Solving the Problems]
In claim 1 of the overcurrent protection circuit according to the present invention,
In the overcurrent protection circuit that protects the battery pack when the load is short-circuited,
The positive electrode of the storage battery pack and the load are connected via the internal resistance of the storage battery pack,
An FET having a source connected to the negative electrode of the battery pack, a drain connected to the load, and a gate connected to the positive electrode of the battery pack via a first resistor;
A capacitor connected in parallel with the resistor;
The input terminal is connected to the drain of the FET through a second resistor, the emitter of the output transistor is connected to the source of the FET, and the collector of the output transistor is connected to the gate of the FET. An amplifying composite circuit including a transistor;
With
When the output voltage of the battery pack suddenly drops due to a short circuit,
In a very short time, the change in the output voltage is transmitted to the gate of the FET through the capacitor to activate the current limit by the FET,
The output transistor of the amplification composite circuit is turned on by current limiting by the FET,
By turning on the output transistor of the composite circuit for amplification, the FET is turned off, and the current flowing through the load is cut off.
In claim 2,
The overcurrent protection circuit according to claim 1 is arranged in two stages between the battery pack and the load, the source of the FET of the first stage overcurrent protection circuit, and the FET of the second stage overcurrent protection circuit Connected to the drain .
In claim 3,
The emitter of the output transistor of each amplifying composite circuit included in the overcurrent protection circuit according to claim 2 is connected to the source of the FET of the overcurrent protection circuit connected next to the storage battery pack. The input terminal of each amplification composite circuit included was connected to the drain of the FET of the overcurrent protection circuit connected next to the load via a second resistor.
In claim 4,
The composite circuit for amplification of the overcurrent protection circuit according to claims 1 to 3 is constituted by a single transistor.
In claim 5,
A Zener diode is further provided between the gate and the source of the FET of the overcurrent protection circuit according to claim 1.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an overcurrent protection circuit according to the present invention will be described in detail with reference to the drawings showing embodiments thereof.
[0008]
In FIG.
Reference numeral 1 denotes a storage battery pack (rated output voltage = 7.2 V) using, for example, a lithium ion (Li-ion) battery. For example, 2 is a load of a wireless device or the like.
Reference numeral 3 denotes an overcurrent protection circuit according to the present invention, which is configured as follows.
Q1 is an FET inserted between the negative pole of the storage battery pack 1 and the negative pole of the load 2. 5 is a Zener diode (Vz = 5.6 V) connected in parallel to the gate and source of the FET , R1 is a first resistor inserted between the positive electrode of the battery pack 1 and the gate of the FET , and C is A capacitor connected in parallel to the first resistor R1. Through the parallel circuit of the first resistor R1 and the capacitor C, the gate of the FET is connected to the positive electrode of the storage battery pack.
The positive electrode of the storage battery pack 1 is connected to the load 2.
[0009]
Q2 is an amplifying composite circuit including an open collector type output transistor, the input terminal is connected to the drain of the FET via a second resistor R2, and the emitter of the output transistor serving as the ground terminal is the source of the FET . The collector of the output transistor is connected to the gate of the FET .
R represents the internal resistance of the storage battery pack 1.
[0010]
A parallel circuit of the first resistor R1 and the capacitor C is set to transmit a change in load voltage to the gate of the FET in a very short time . For example, the first resistor R1 is set to 470 KΩ, and the capacitor C is set to 0.11 μF.
Various secondary batteries can be used as the storage battery pack 1, and further, a battery pack using a primary battery can be used instead of the storage battery pack 1. In addition, the battery pack includes a case where the battery is used alone.
[0011]
In the overcurrent protection circuit configured as described above,
In the steady state, since the gate voltage of the FET is 5 V or more, the FET is turned on, and the voltage of the storage battery pack 1 is normally applied to the load 2.
When the load is short-circuited, an overload current of, for example, 20 A or more flows instantaneously to the load 2, so that the output voltage of the storage battery pack 1 rapidly decreases by about 3V due to a voltage drop due to the internal resistance R of the storage battery pack 1. To do.
Abrupt changes in the output voltage of the battery pack 1, Runode transmitted to the gate of the FET in a very short time by a parallel circuit of a capacitor C and a first resistor R1, the gate voltage of the FET is reduced to about 2V.
[0012]
Since the resistance value between the source and drain of the FET increases as the gate voltage of the gate of the FET decreases, the overload current flowing through the load is limited.
By the resistance value between the source and drain of the FET increases, the voltage of the input end terminal of the amplifying composite circuit Q2 a voltage drop due to the internal resistance of the FET is increased to increase.
Therefore, the output transistor of the amplifying composite circuit Q2 is turned on, and the gate voltage of the FET becomes 0 V, which is the same as the drain voltage. Therefore, the FET is turned off and no current flows through the load.
Thus, since the overload current is limited in a very short time, the occurrence of sparks can be prevented and the battery pack can be reliably protected.
[0013]
As described above, the load voltage change is transmitted to the gate of the FET in a very short time by the parallel circuit including the capacitor C, and the current limitation by the FET is once activated.
Since the current is limited , the voltage at the input terminal rises, so that the output transistor of the amplifying composite circuit Q2 is turned on.
Subsequently, when the amplifying composite circuit Q2 is turned on, the FET is turned off to cut off the current.
[0014]
When short circuit load is released, the voltage of the input terminal is lowered to near 0V, the output transistor of the amplifier composite circuit Q2 is turned off, the gate voltage of the FET is increased, the FET is turned on, Return to steady state.
[0015]
In this way, according to the overcurrent protection circuit 3 shown in FIG. 1, when the load 2 is short-circuited, the overload current is cut off in an extremely short time, so that the battery pack is protected and the spark Occurrence can be prevented. Therefore, it is easy to configure an electronic device that satisfies a predetermined explosion-proof standard.
[0017]
Na us, constant of the parallel circuit constituted by the first resistor R1 and the capacitor C can be selected various constants.
Then, it is also be constructed using a single transistor in place of the amplifier composite circuit Q2. The amplifying composite circuit Q2 may be any circuit or element having a function of turning on when the voltage drop in the FET increases and shifting the FET to the off state.
[0018]
In addition, as shown in FIGS.
The overcurrent protection circuits 3 and 3 having the above configuration are arranged in two stages, the source of the FET of the first stage overcurrent protection circuit is connected to the drain of the FET of the second stage overcurrent protection circuit, and the load Only the drain of the FET of the overcurrent protection circuit connected next to the load may be connected to the load, and only the source of the FET connected next to the storage battery pack may be connected to the negative electrode of the storage battery pack (FIG. 2). reference). Alternatively, the emitter of the output transistor of each amplification composite circuit included in the overcurrent protection circuit arranged in two stages as described above is connected to the source of the FET of the overcurrent protection circuit connected next to the storage battery pack, The input terminal of each amplifying composite circuit included in the overcurrent protection circuit may be connected to the drain of the FET of the overcurrent protection circuit connected next to the load via the second resistor. Yes (see FIG. 3). By adopting these configurations, it is possible to reliably cut off the load current even if a malfunction occurs due to the failure of one of the circuits, and it is possible to configure an electronic device that satisfies a more advanced explosion-proof standard. can Ru.
[0019]
【The invention's effect】
According to the overcurrent protection circuit of the present invention, since the current limitation by the FET is started in a very short time after the load is short-circuited , the current flowing to the load can be limited in a very short time, thereby preventing the occurrence of sparks. The output transistor of the amplifying composite circuit can be turned on by starting the current limitation, and the current can be cut off by turning the FET off. That is, it is possible to limit the overload current of the battery pack in a very short time and to prevent the occurrence of sparks, and it is possible to provide an overcurrent protection circuit that operates at high speed with a small number of components.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram of an embodiment of an overcurrent protection circuit according to the present invention.
FIG. 2 is a circuit configuration diagram of another embodiment.
FIG. 3 is a circuit configuration diagram of still another embodiment.
FIG. 4 is a circuit configuration diagram of a conventional example.
[Explanation of symbols]
1 battery pack, battery pack 2 radio, load 3 overcurrent protection circuit Q1 FET
5 Zener diode R1 first resistor C capacitor <br/> Q2 open collector type amplifier composite circuit including an output transistor

Claims (5)

In the overcurrent protection circuit that protects the battery pack when the load is short-circuited,
The positive electrode of the storage battery pack and the load are connected via the internal resistance of the storage battery pack,
An FET having a source connected to the negative electrode of the battery pack, a drain connected to the load, and a gate connected to the positive electrode of the battery pack via a first resistor;
A capacitor connected in parallel with the resistor;
The input terminal is connected to the drain of the FET through a second resistor, the emitter of the output transistor is connected to the source of the FET, and the collector of the output transistor is connected to the gate of the FET. An amplifying composite circuit including a transistor;
With
When the output voltage of the battery pack suddenly drops due to a short circuit,
In a very short time, the change in the output voltage is transmitted to the gate of the FET through the capacitor to activate the current limit by the FET,
The output transistor of the amplification composite circuit is turned on by current limiting by the FET,
An overcurrent protection circuit configured to turn off the FET by turning on an output transistor of the amplifying composite circuit to cut off a current flowing through a load. The overcurrent protection circuit according to claim 1 is arranged in two stages between the battery pack and the load, the FET source of the first stage overcurrent protection circuit, and the FET of the second stage overcurrent protection circuit An overcurrent protection circuit, characterized in that the drain is connected. The emitter of the output transistor of each amplification composite circuit included in the overcurrent protection circuit according to claim 2 is connected to the source of the FET of the overcurrent protection circuit connected next to the storage battery pack. An overcurrent protection circuit, wherein the input terminal of each of the included composite circuits included is connected to the drain of the FET of the overcurrent protection circuit connected next to the load via a second resistor. 4. The overcurrent protection circuit according to claim 1, wherein the amplifying composite circuit is constituted by a single transistor. The overcurrent protection circuit according to claim 1, further comprising a Zener diode between a gate and a source of the FET.

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